1. Field of the Invention
This invention relates to the apparatus and method utilizing ultra-high frequency or microwave energy for the purpose of measuring the humidity in a gas such as air.
2. Description of the Prior Art
Instruments of humidity measurement, generally known as hygrometers, are many in kind and have long been in use. Examples are:
1. A hair hygrometer, in which measurement is based on the contraction and extension of hair in proportion to changes in humidity; PA1 2. a dew point hygrometer, which produces a humiditiy measurement by directly detecting the dew point of a gas under investigation; PA1 3. a psychrometer, which measures the weight of the water vapor contained in a gas; PA1 4. Assamann's aspiration psychrometer, which provides humidity measurements by aspiration; PA1 5. an absorption hygrometer, in which humidity measurements are produced from the change in electric resistance of a hygroscopic material with humidity.
It is well known that the above-mentioned instruments are not accurate in measurement, unreliable in measuring operation and difficult to maintenance properly. The humidity information made available from these instruments is not in the form of electrical signal and, therefore, cannot be utlized as an input signal to any automatic control device.
Recently a humidity measurement system was proposed. It is a system employing two cavity resonators. Its measuring operation is based on the fact that the humidity of air effects the transmission characteristic of an ultra-high-frequency signal of the kind commonly known as microwave. The gas, whose humidity is to be checked, is contained in one resonator and dry air is contained in the other. A microwave of the desired mode and frequency is fed into the two resonators to produce a resonant condition in each. Since the amount of water contained in the gas under investigation or, simply, the humidity increases the dielectric constant or permittivity of of the gas, there occurs a difference in resonant frequency between the two cavity resonators. This difference, that is, "beat" frequency is measured to tell the humidity in the gas.
This recently proposed system too has drawbacks in that the detector circuitry for finding out the value of said beat frequency is necessarily complex to result in a high manufacturing cost, and that the temperature changes of the two resonators must be taken into consideration to accurately measure the respective resonant frequencies if the measuring error due to temperature changes is to be reduced to an extent necessary for practical humidity measurement. Thus, the devices for this system are inevitably complicated and expensive.